Most engines comprise a number of rotating parts, and thus have an ideal speed range in which their power output is optimum. This ideal range may correspond, for example, to a range exhibiting peak torque output as a function of RPM. A broader permissible speed range encompasses this ideal range and includes greater and lesser speeds at which the engine may operate, even if in a suboptimum manner. Finally, outside of this permissible speed range lie speeds at which the engine cannot provide sustained operation. For example, speeds that are higher than the highest speed in the permissible range may cause greatly accelerated or catastrophic failure of the engine, transmission, or implement system.
At speeds lower than the bottom limit of the larger range, the engine may cease rotation. In particular, most engines operate via an inertia-driven cycle, wherein preceding combustion events power the engine toward subsequent combustion events via the engine's rotational inertia. When the engine speed decreases below a certain lower limit, the engine's rotational inertia is insufficient for the engine to reach subsequent combustion events. An example of a lowest reliable operating speed based on this principle is the engine “idle” speed. Typically, lower engine speeds are possible, but the idle speed is set to a value that allows for a slight decrease in engine speed without causing the engine to drop out of the sustained combustion range.
In a typical machine arrangement, the engine inertia must be sufficient to overcome not only the internal resistance leading up to a subsequent combustion event, but also any outside resistance imposed by the power train. For example, the inertial, frictional, or other resistance involved in moving the machine must be overcome when the machine is in gear. Thus, while the idle speed is a realistic lower limit when the machine is stationary, a machine in operation may have a heightened lower limit, below which the engine lacks sufficient power to accelerate or even continue a present operation. When the engine speed drops past this lower limit, the engine is said to “lug” down or “bog” down, and continued reliable operation is jeopardized.
In a conventional-drive machine, the engine is generally linked to the power train and other power sinks of the machine via a torque converter. In these systems, a higher resistance (required torque) is automatically mitigated by the natural loading characteristics of a torque converter, thus preventing the engine from lugging down and stalling. However, in a CVT-driven machine (“CVT” denotes a continuously variable transmission), there is generally no torque converter, and the machine resistance will be able to lug down and stall the engine absent an external control mechanism. Typically, the engine is monitored for lug/stall problems and the transmission is actively controlled, e.g., via a software Engine Underspeed Algorithm (EUA) in an Electronic Control Module (ECM) to avoid lug/stall and to force the engine to operate at the desired optimal speed condition. In other words, the EUA acts to prevent stalling and then may also act to return the engine to its peak power point.
A typical EUA reduces the drivetrain transmission speed demand, in reaction to a difference between the actual engine speed and the desired engine speed (e.g., “speed standard”), detected from a user interface or from an engine control component as a response to changed conditions.
Although the resolution of deficiencies, noted or otherwise, of the prior art has been found by the inventors to be desirable, such resolution is not a critical or essential limitation of the disclosed principles. Moreover, this background section is presented as a convenience to the reader who may not be of skill in this art. However, it will be appreciated that this section is too brief to attempt to accurately and completely survey the prior art. The preceding background description is thus a simplified and anecdotal narrative and is not intended to replace printed references in the art. To the extent an inconsistency or omission between the demonstrated state of the printed art and the foregoing narrative exists, the foregoing narrative is not intended to cure such inconsistency or omission. Rather, applicants would defer to the demonstrated state of the printed art.